Pressure support ventilation of patients

ABSTRACT

A methodology and apparatus for determining ventilator settings including an end expiratory pressure setting, pressure swing, resistive unloading and target ventilation for delivering ventilatory support based upon generalized patient ventilation characteristics and/or disease classifications. An apparatus may be programmed with the instructions to accomplish the methodology interactively by prompting the user/physician during setup and calculating settings based upon measurements or input responses. Pre-assigned values associated with ventilation characteristics or disease classifications may be combined with a base pressure value or measured values to provide patient customized settings or adjustments to determine pressure levels for the delivery of ventilatory support.

[0001] This application claims the priority filing date of U.S.provisional patent application serial No. 60/306,972 filed on Jul. 19,2001.

FIELD OF THE INVENTION

[0002] This invention relates to methods and devices for providingventilatory assistance to a patient. More specifically, the inventioninvolves an improved method and device for adjusting the device settingsto provide ventilation to satisfy a patient's respiratory needs.

BACKGROUND OF THE INVENTION

[0003] In untreated patients with lung, chest wall, or controlabnormalities, blood gases typically deteriorate somewhat in NREM sleep,and then deteriorate much further in REM sleep. This deterioration islikely due to multiple causes, including:

[0004] 1. Increased upper airway resistance due to pharyngeal collapse.

[0005] 2. Loss of cough and sigh, leading to sputum retention andatelectasis.

[0006] 3. Postural effects on V/Q.

[0007] 4. Reduced tonic or chemoreflex drive to the diaphragm,particularly in REM.

[0008] 5. Reduced tonic or chemoreflex drive to intercostals, abdominalexpiratory muscles, and other accessory muscles.

[0009] 6. Possible REM-specific changes in pulmonary blood flowdistribution.

[0010] Pharyngeal collapse is most profound in REM sleep. There isevidence that the reduction in ventilation in NREM sleep is entirely dueto pharyngeal collapse, and not to a reduction in chemoreflex drive tothe diaphragm. Increased pharyngeal resistance is treated with CPAP, ormore generally with positive pressure sufficient to splint the airway atzero flow, plus additional inspiratory pressure sufficient to compensatefor resistive and Bernoulli pressure drop.

[0011] Reducing the work of breathing and resting the respiratorymuscles by providing ventilatory support, particularly if deliveredduring sleep, can have a number of direct and indirect potentialbenefits. These benefits include:

[0012] Prevention of muscle fatigue with inefficient contraction.

[0013] Reduced oxygen cost of breathing.

[0014] Reduction of dyspnea.

[0015] Improved sleep, with fewer respiratory arousals.

[0016] Improved sleep should in turn reduce metabolic rate, CO₂production and oxygen consumption, directly and indirectly by reducedrolling around, fidgeting, etc., leading to either better blood gases orreduced need for ventilatory support. It is also worthwhile in its ownright because of improved quality of life.

[0017] However, there are some untoward effects of ventilatory supporton the patient as follows:

[0018] 1. Barotrauma

[0019] For ventilators delivering less than 35 cmH₂O peak pressure,barotrauma is largely confined to patients with adult respiratorydistress syndrome (due to high shear stresses) and to patients with ahistory of pneumothorax or emphysematous bullae.

[0020] 2. Reduced cardiac output

[0021] Even in normal subjects, 10 cmH₂O nasal CPAP can produce a 10%reduction in cardiac output, and high levels of positive pressure,particularly in patients who are fluid depleted, can produce a profoundreduction in cardiac output. Conversely, in patients with cardiacfailure and fluid overload (pulmonary capillary wedge pressure in excessof 15 cmH₂O), nasal CPAP actually increases cardiac output, probably byreducing transmural pressure.

[0022] 3. Mouth leak

[0023] Mouth leak is present to some extent in most patients beingtreated with ventilatory support. A mouth leak of 0.4 L/sec causessevere sleep disruption, loss of ventilatory support, loss ofsupplemental oxygen, and loss of end expiratory splinting pressure. Sucha leak is present in perhaps 50% of subjects. Mouth leak also causesincreased nasal resistance. This is a reflex response to drying andcooling of the nasal mucosa by a unidirectional flow of air in the noseand out the mouth.

[0024] A chin strap is only very partially effective in controllingmouth leak. Heated humidification can partially treat the, drying of thenasal mucosa but not the other aspects of the problem. Where tolerated,a full face mask is the preferred treatment.

[0025] 4. Glonic closure

[0026] Rodenstein and colleagues have shown that over ventilation leadsto a progressively tight closure of the vocal cords, both awake andasleep, and that this fact must be taken into account when providingnoninvasive ventilation.

[0027] The details are not well understood; it is not known whether theglottic closure is purely passive or involves active adduction, whetherit is abolished by anaesthesia, whether it is present in REM, whether itis due to airway or arterial hypocapnia, or whether it is produced bysleepstate specific changes in set-point. Unlike passive pharyngealcollapse, it is not known whether vocal cord closure responds to CPAP,but if it is an active closure it would be expected to be extremelyrefractory to CPAP.

[0028] 5. Increased deadspace

[0029] Positive pressure will alter the distribution of pulmonary bloodflow, tending to reduce blood flow to poorly ventilated units(beneficial reduction in physiological shunt) and also towell-ventilated units (pathological increase in deadspace). In patientsin whom there is much blood flow to poorly perfused lung units, forexample patients with obesity hypoventilation syndrome, this reductionin physiological shunt but increase in deadspace can be of net benefit,whereas in patients with much ventilation to poorly perfused regions,such as “pink puffers”, the net effect can be detrimental.

[0030] 6. Discomfort

[0031] A goal of a ventilator is to relieve dyspnea. However, it cancause considerable discomfort, by various mechanisms:

[0032] Distension of upper airway structures.

[0033] Swallowing of air (particularly once pressures exceed 20 cmH₂O)

[0034] Mask discomfort.

[0035] Leak, particularly mouth leak.

[0036] Patient-machine asynchrony.

[0037] We might expect that as the degree of support is increased fromzero towards that which will perform 100% of eupneic respiratory work,the sense of dyspnea due to having to do an abnormally high amount ofrespiratory work, and the sense of distress due to excess chemoreflexstimulation should both decrease towards zero. However, discomfort fromall the causes bulleted above will increase. There is no literature onthe rate of trade-off between the two sources of distress, but it isapparent that the patient should feel most comfortable at a degree ofsupport which is less than 100% support. Very preliminary unpublishedwork by the current author, in which normal subjects breathe through ahigh external resistance (8 cmH₂O/L/sec) with 200 ml added deadspace,and are then treated with bilevel support, the patient feels mostcomfortable at about 50% support. The optimum point may of course bequite different in a patient with actual lung or chest wall disease, orwith forms of support other than bilevel.

[0038] 7. Patient-machine asynchrony

[0039] Patient-machine asynchrony can be due to a number of factors,including:

[0040] Leaks.

[0041] Long respiratory time constant (e.g. in patients with severechronic airflow limitation (“CAL”).

[0042] Intrinsic PEEP.

[0043] Leaks, and particularly variable leaks, cause asynchrony becausethe airflow measured by the device does not equal the patientrespiratory airflow. With a device of the invention, leaks start tobecome a problem at about 0.2 L/sec, and are a severe problem by 0.4L/sec. At 0.6 L/sec, the device will probably not really be benefitingthe patient. Keeping the leak much below 0.2 L/sec is technically verydemanding and not generally practicable. Therefore, while one wants tokeep the leak as low as possible with reasonable investment of effort,0.2 L/sec is a reasonable balance between effort and results.

[0044] Patient-machine asynchrony is particularly a problem in patientswith long respiratory time constants being treated with high degrees ofsupport. This is because even true respiratory airflow no longer equalspatient effort. For example, at the end of the patient's inspiratoryeffort, the lungs have not yet equilibrated to the high inspiratorypressure and continue to fill. This prevents correct triggering intoexpiration. The patient must actively expire in order to terminate theinspiration. The higher the degree of support results in greaterdifficulty with the phenomenon. Therefore, one wants to avoid excessivesupport.

[0045] Intrinsic PEEP causes a kind of asynchrony because the patientmust generate a considerable inspiratory effort before any flow isgenerated. Intrinsic PEEP due to dynamic airway compression may beevident from an expiratory flow-time curve, in which there is a briefperiod of very high expiratory flow, followed by a very prolongedexpiratory flow plateau at a much lower flow. Treatment is to increaseexpiratory pressure (particularly late expiratory pressure) until thecurve shape normalizes.

[0046] Thus, with these seven effects in mind, the goals of automaticventilatory positive airway pressure may generally be summarized toinclude the following:

[0047] 1. To guarantee an adequate alveolar ventilation during sleep.

[0048] 2. To maximize wake comfort.

[0049] 3. To maximize depth of sleep.

[0050] 4. To minimize cost of initiation of therapy.

[0051] Directed towards the above goals, a ventilator device inaccordance with the invention may provide:

[0052] 1. Servo-control of minute ventilation to equal or exceed achosen target.

[0053] 2. Unloading of much of the spontaneous resistive work if thesubject exceeds the chosen target.

[0054] 3. A smooth and physiological pressure waveform whose minimumamplitude will unload much but not all of spontaneous elastic work ifthe subject just exceeds the chosen target.

[0055] 4. A mechanism for automatically establishing the target duringan awake learning session in subjects who have adequate PCO₂ in thedaytime and who deteriorate only during sleep.

[0056] However, even sophisticated ventilatory devices with a highdegree of automatic processing developed to meet one or more of thesegoals such as the devices disclosed in Intemational Publication No. WO98/12965 and International Publication No. WO 99/61088 still oftenrequire the setting of controls to accommodate a particular patientsneeds before beginning use. Absent a uniform methodology for adjustingthe settings of such a device, the delivery of the appropriate degree ofpressure support to the patient may not be optimal.

BRIEF DESCRIPTION OF THE INVENTION

[0057] Accordingly, keeping with the above goals and/or other goals thatwill be apparent to those skilled in the art, the invention is a novelmethodology for adjusting the settings of a ventilator. In one form ofthe invention, a pressure setting to maintain a positive end expiratorypressure is determined using assigned adjustment pressure valuesrepresenting generalized patient ventilation characteristics such asobesity, sleepiness, chronic airflow limitation, etc. As a result ofresponses to inquiries, the assigned adjustment pressures are added to astarting or default pressure setting. The starting pressure setting ispreferably about 4 cmH₂O and the adjustment pressures preferably rangefrom about 1-2 cmH₂O. The resulting range is about 4-10 cmH₂O. Supportpressure may then preferably be delivered in accordance with a pressureformula that accounts for resistive unloading and a determinedrespiratory phase as a continuous phase variable. The methodology may beimplemented by an apparatus programmed to execute the methodologyinteractively by prompting a user/physician to respond to thepredetermined inquiries and then calculate the adjustment based upon theinput responses.

[0058] Therefore, the invention includes a method or apparatus fordetermining a setting for a ventilator to deliver support to a patientto maintain a positive end expiratory pressure comprising the steps ofselecting an initial pressure value; prompting for responses to queriesabout a patient concerning generalized ventilation characteristics; andcalculating a positive end expiratory pressure from said initialpressure value and a set of adjustment pressure values based upon saidresponses to said queries, said adjustment pressure values representinggeneral ventilation characteristics.

[0059] A further embodiment of the invention involves determining asetting for a ventilator to deliver support to maintain a pressure swingin a specified range. The swing is preferably chosen to do about 50% ofa patients elastic work. In the method an initial pressure value isselected and based upon responses prompted to determine degrees ofseverity, for example, mild, moderate and severe, of restrictivemechanical abnormality of the lung or chest wall of a patient, apressure swing setting is calculated with the initial pressure andpressure values assigned to the different degrees of severity. Thepreferred assigned values in a range of about 2-6 cmH₂O lead to a swingof about 5-9 cmH₂O. As with the other embodiments of the invention, themethodology may be manual or implemented via interactive responses toprompts issued from an automated apparatus.

[0060] Therefore, the invention includes a method or apparatus fordetermining a setting for a ventilator to deliver support to maintain apressure swing in a specified range chosen to do about half of apatient's elastic work comprising the steps of selecting an initialpressure value; prompting for a response to a query about a patientconcerning degrees of severity of a restrictive mechanical abnormalityof lung or chest wall; and calculating a pressure swing from saidinitial pressure vaiue and a set of adjustment pressure values basedupon said response to said query, wherein said set of adjustmentpressure values represent degrees of severity of a restrictivemechanical abnormality of lung or chest wall.

[0061] Another embodiment of the invention involves a methodology fordetermining resistive unloading for a ventilator setting to deliversupport. The setting is preferably chosen to unload about 50% to 80% ofa subject's resistive work. The method involves the use of assignedresistive unloading pressure values representing degrees of severity ofone or more diseases, for example, restrictive disease and/orobstructive disease. By prompting for a response to determine whetherthe patient suffers from a particular degree of severity, for example,mild, moderate or severe, the setting can be determined from theresponse by using the assigned pressure value. In the preferredembodiment, the pressure values for mild, moderate and severeobstructive disease are in a range of about 4-8 cm H₂O/L/sec. andpreferably 4, 6 or 8 cmH₂O respectively. The pressure values for mild,moderate and severe restrictive disease are in a range of about 3-8 cmH₂O/L/sec. and preferably 3, 6 or 8 cmH₂O respectively. The method maybe performed manually. Alternatively, a ventilator device is programmedwith the instructions to accomplish the method interactively.

[0062] Therefore, the invention includes a method or apparatus fordetermining a resistive unloading setting for a ventilator to deliversupport to unload about 50% to 80% of a subject's resistive workcomprising the steps of prompting for a response to at least one queryto determine the subject's degree of severity of restrictive disease andobstructive disease; and setting a resistive unloading value to one of aset of assigned pressure values based upon said response to said atleast one query, wherein said set of assigned pressure values representsdegrees of severity of restrictive disease and obstructive disease.

[0063] In one form of the invention, a target ventilation setting isdetermined as a function of measured PCO₂. In the invention, a patient'sventilation is measured over time during a learning period in whichventilatory support is provided and a target ventilation is derived as afunction of the measured ventilation. The patients arterial partialpressure of CO₂ is also measured. The target ventilation is thenadjusted as a result of the measured arterial partial pressure.Preferably, the measure is compared to a threshold PCO₂ and the targetventilation may be increased or decreased based upon the comparison. Thetarget ventilation may then be increased or decreased based upon a fixedpercentage of the absolute value of the difference between the measuredPCO₂ and the threshold PCO₂. In the preferred calculation, the thresholdis about 50 mmHg.

[0064] Therefore, the invention includes a method or apparatus fordetermining a target ventilation setting of a ventilator comprising thesteps of delivering ventilatory support during an awake learning periodto a patient; measuring the patient's ventilation over time during alearning period; measuring the patients partial pressure of CO₂;calculating a target ventilation as a function of a measure ofventilation; and adjusting said target ventilation as a function of ameasure of partial pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0065]FIG. 1 depicts a ventilator apparatus operable for performing themethodology of the invention;

[0066]FIG. 2 depicts a typical example of a pressure component of thedelivery pressure equation n accordance with the invention;

[0067]FIG. 3 illustrates a waveform template in accordance with theinvention;

[0068]FIG. 4 shows an actual pressure waveform of the delivered pressurefrom an apparatus of the invention;

[0069]FIG. 5 is a flow chart illustrating steps in a method fordetermining an end expiration pressure setting for a ventilator of theinvention;

[0070]FIG. 6 is a flow chart illustrating steps in a method fordetermining a resistive unloading pressure setting for a ventilator ofthe invention;

[0071]FIG. 7 is a flow chart illustrating steps in a method fordetermining a resistance swing pressure setting for a ventilator of theinvention;

[0072]FIG. 8 is a flow chart illustrating steps in a method fordetermining a target ventilation setting for a ventilator of theinvention;

DETAILED DESCRIPTION OF THE INVENTION

[0073] A servo-controlled ventilator useful for accomplishing theinvention is shown in FIG. 1. A blower 10 supplies air under pressurevia delivery tube 12 to a mask 11 or via another such device forproviding flow to a patient's respiratory system. Exhaust gas is ventedvia exhaust 13. Mask flow is preferably measured using pneumotachograph14 and differential pressure transducer 15 to derive flow signal f(t).Mask pressure is measured at pressure tap 17 using pressure transducer18. Flow and pressure signals are sent to a controller or microprocessor16 including a memory which implements the processing described hereinto derive a pressure request signal P(t). Programmed instructionsaccessible to the microprocessor are coded on integrated chips in thememory of the device or may be loaded as software and stored by someother data storage medium of conventional design (not shown). The actualmeasured pressure and pressure request signal P(t) are fed to motorservo 19 which controls blower motor 20 to produce the desiredinstantaneous mask pressure. Optionally, an automated PCO₂ measurementdevice 21 or other non-invasive blood gas monitor/device for measuringPCO₂ may be linked to provide an input data signal to the microprocessor16, for example, a device as taught in U.S. patent Ser. No. 5,630,413,the disclosure of which is incorporated by reference. Optional inputand/or output devices 22 may be included to display output signals andenter input signals for the microprocessor 16. Various appropriate inputand output devices such as keypads and display screens and otheralternatives are known in the art.

[0074] An example of this type of servo-controlled ventilator is thesubject of International Publication No. WO 98/12965, which is alsodisclosed in related U.S. application Ser. No. 08/935,785. An additionalexample is disclosed in International Publication No. WO 99/61088, whichis also contained in related U.S. application Ser. No. 09/316,432. Theforegoing U.S. applications are hereby incorporated by reference.

A. PRINCIPLES OF OPERATION

[0075] The goals of automatic ventilatory positive airway pressuredevice of the invention (“AutoVPAP”) are:

[0076] 1. To guarantee an adequate alveolar ventilation during sleep.

[0077] 2. To maximize wake comfort.

[0078] 3. To maximize depth of sleep.

[0079] 4. To minimize cost of initiation of therapy

[0080] and that, directed towards the above goals, a device inaccordance with the invention provides:

[0081] 1. Servo-control of minute ventilation to equal or exceed achosen target.

[0082] 2. Unloading of much of the spontaneous resistive work if thesubject exceeds the chosen target.

[0083] 3. A smooth and physiological pressure waveform whose minimumamplitude will unload much but not all of spontaneous elastic work ifthe subject just exceeds the chosen target.

[0084] 4. A mechanism for automatically establishing the target duringan awake learning session in subjects who have adequate PCO₂ in thedaytime and who deteriorate only during sleep.

[0085] 1. Servo-ventilation: Choosing a Target

[0086] A servo-ventilator can guarantee a minimum ventilation, andthereby prevent the component of REM hypoxia due to hypoventilation. Themethodology for choosing the target ventilation may depend upon thepatients condition.

[0087] 1. Acutely decompensated subjects

[0088] In subjects who are acutely decompensated, or in whom the daytimePCO₂ is unacceptable, it is necessary for the clinician to empiricallydetermine a target ventilation, for example, by starting at 70ml/Kg/min, and adjusting according to blood gases.

[0089] 2. Chronically stable subjects

[0090] In subjects in whom the daytime awake PCO₂ is perhaps not perfectbut at least adequate, the device provides a simple facility forautomatically determining a target ventilation. Briefly, the devicemeasures the subject's spontaneous ventilation during a partiallyassisted awake acclimatization session, and sets the target ventilationto equal 90% of the median ventilation during the final 40 minutes ofthe session. The backup respiratory rate (to be used only if the subjectfails to be adequately ventilated) is set to the median respiratory rateduring the acclimatization session.

[0091] The rationale for setting the target ventilation to 90% ratherthan 100% of the median awake ventilation is that there will be a 10-15%reduction in metabolic activity during sleep. If the ventilation is setto 90% of the daytime awake ventilation, then to a first approximation,the overnight PCO₂ will be held at close to the daytime awake PCO2during the acclimatization session. Conversely, if the target were setto 100% of the spontaneous awake ventilation, then the patient would bedriven to central apnea in NREM sleep. This would produce active vocalcord adduction, resulting in unnecessary delivery of maximum pressure.

[0092] 2. Equation for Mask Pressure

[0093] The instantaneous mask pressure is set according to the followingequation:

P=P _(eep) +R·f+A·Π(φ)

[0094] where:

[0095] P_(eep) is the pressure at end expiration, used to splint theupper airway, unload intrinsic PEEP, and reduce atelectasis. (Itcorresponds very loosely with EPAP on a bilevel ventilator.)

[0096] f is the respiratory airflow.

[0097] φ is the instantaneous phase in the respiratory cycle.

[0098] R is a resistance equal to about 50-80% of the patient's actualairway resistance, and will generally be in the range 2 to 8cmH₂O/L/sec. The R·fterm is independent of any estimation of phase, andhelps to provide good patient-machine synchronization at the criticalmoments of start of inspiration and start of expiration. A typicalexample of the pressure component due to this term for R=6 cmH₂O/L/secis shown in FIG. 2.

[0099] A is the difference between pressure at end inspiration andpressure at end expiration. (It corresponds loosely to the differencebetween IPAP and EPAP on a bilevel ventilator.)

[0100] Π(φ) is a pressure waveform template which, providing the patientis being ventilated at or above the target ventilation, is shown in FIG.3. It should be noted that the pressure waveform template is flat (nochange with time) at three places: at the start of inspiration (φ=0),just before end inspiration (φ=0.5), and at end expiration (φ=1.0). Theeffect is to make the estimated phase have very little effect onpatient-machine synchronization at these critical points.

[0101] The pressure modulation amplitude, or swing, A, is automaticallyadjusted between a physician-selectable maximum and minimum suing,A_(max) and A_(min), respectively, using the following equation:$A = {{{- G}{\int\frac{f}{2}}} - {V_{TGT}{t}}}$

[0102] Where V_(TGT) is the chosen target ventilation, and G is theservo-controller gain, which is set to 0.3 cmH₂O increase in support persecond for every L/min error in ventilation. If the patient is breathingat above the chosen target ventilation, then the degree of support willfall to the physician-selected minimum swing A_(min). Conversely, if thesubject is breathing at less than the target ventilation, the degree ofsupport will increase quite rapidly until either the target ventilationis reached, or until the degree of support reaches A_(max).

[0103] Once the degree of support reaches A_(MAX), the shape of thepressure waveform template becomes progressively more square, andtherefore more efficient at generating flow, until either the targetventilation is reached or the waveform is maximally square. Thus,AutoVPAP will try initially to treat the patient with a smooth andcomfortable waveform, but if this does not work, it uses a progressivelymore aggressive waveform, until it succeeds.

[0104] The combination of all the terms produces a waveform typicallylike that shown in FIG. 4.

[0105] 3. Phase

[0106] AutoVPAP uses a 14-rule fuzzy logic algorithm to determine theinstantaneous phase φ in the machine respiratory cycle. Firstly, thereare a series of rules which infer the machine phase from the patientsrespiratory airflow, attempting to synchronize directly with thepatient. These rules are most strongly active if the patient isbreathing at or above the physician-prescribed target ventilation, andthe leak is small and steady, but the rules are only weakly active ifthere is hypopnea or a large or changing leak.

[0107] Another rule says that the rate of change of phase equals thepatient's recent observed respiratory rate (which is different forinspiration and expiration, to allow for differing times for inspirationand expiration (T_(I) and T_(E)). This rule allows AutoVPAP to learn thepatient's typical respiratory rate and duty cycle. It is also mostactive if the patient is breathing at or above target, and weak if thereis hypopnea or leak.

[0108] Finally, there is a rule which says that the phase is increasingat the physician-set backup respiratory rate. This rule is normallyalmost inactive, but if the ventilation starts to fall below the target,or if there is a long expiratory pause, the rule becomes rapidly moreactive, hastening the next machine breath.

[0109] The net effect of ail the rules is that most of the time, whenthe patient is making reasonable efforts of his own, amplified andaugmented by the machine efforts, so that the minute ventilation is ator above target, the machine will synchronize very accurately with thepatient.

[0110] Conversely, if the patient is making only feeble efforts (roughlyspeaking, the patients transdiaphragmatic pressure swing is less thanabout 25% of the machine's pressure swing) the device will no longer beable to always synchronize with the patient.

[0111] Even if the patient is centrally apneic, the backup rate will notnecessarily be used. The machine may ventilate the patient either fasteror slower than the backup rate, depending on lung and chest wallmechanics. There is a tendency for AutoVPAP to use very slow, deepbreaths in the face of a high airway resistance, which may beadvantageous if it reduces resistive work and avoids air trapping andintrinsic PEEP.

[0112] The backup rate will only be used if the patient's ventilation isbelow the target ventilation, and the machine cannot give any moresupport by either further increases in swing or by squaring up thewaveform template. Since it is a goal of therapy that the patient'sventilation is never below the target, it follows that the backup rateis rarely used. However, if there is an obstructive apnea, or if thereis closure of the vocal cords, then the backup rate will be used. Thisreluctance to use the backup rate makes AutoVPAP very tolerant of errorsin setting the backup rate.

[0113] 4. Comparison with PAV

[0114] In operation, the apparatus provides superior results compared toproportional assist ventilation devices. Recall that the equation formask pressure with AutoVPAP is:

P=P _(eep) +R·f+A·Π(φ)

[0115] whereas the equation for mask pressure for PAV is:

P=P _(eep) +R·f+E·∫f dt

[0116] The term R·f provides resistive unloading in a manner similar toproportional assist ventilation. However, the remainder of the equationis quite different. The most important consequence is that if thepatient is centrally apneic, PAV provides no support, whereas AutoVPAPprovides increasing support until the target ventilation is achieved.This could potentially be very important for patients with abnormalcontrol of breathing, who could make feeble or no efforts in phasic REMsleep.

B. AUTOVPAP SETUP PROCEDURE

[0117] The steps for setup of the device for an awake leaming period toprecede regular treatment sessions with the device may be outlined asfollows:

[0118] 1. Switching On

[0119] Turn on the blower and computer, connect the two together, andrun the control software, as follows. The order is not critical.

[0120] Connect the blower to the PC using the serial cable provided.(The cable may be extended using a commercial 9 pin male to femaleserial cable with all 9 conductors wired straight through.).

[0121] Switch on the blower, making sure that the patient is notbreathing on the mask, not touching or rattling the hose, and the maskis open to the air (e.g. not blocked by bedclothes etc) or the blowermay fail its self test. Wait for the green “READY” light to come on.

[0122] Start the computer and run the software.

[0123] A few seconds after all steps are completed, flow and pressuredata will appear on the long thin graph across the middle of the screen.The time scale is 0-60 seconds. The respiratory airflow graph scale is 1L/sec (inspiration upwards), and the mask pressure graph scale is 0-25cmH₂O.

[0124] 2. Selecting Initial Settings

[0125] Settings for the machine can be adjusted using a bank of threedouble-sliders labeled EEP, SWING, and PEAK respectively. While thesesliders are virtual controls that are graphically displayed by thecontrol software, optionally, hardware controls can be included tospecify the control settings. The adjustments (1) to (4) below are mosteasily done in the order shown, because the ranges of some settings arelogically determined by others. For example, the sum of the EEP and theswing cannot exceed the maximum peak pressure.

[0126] (1) Mode. If the device has multiple modes, the device should beplaced in an appropriate mode. The “AutoVPAP” mode can be selected byclicking on an AutoVPAP mode icon that is displayed on a display screen.

[0127] (2) Peak and trough pressure. Preferably, the PEAK settingsliders remain at the default values of 22 and 3 cmH₂O respectively

[0128] (3) End expiratory pressure. The EEP setting, (i.e., the P_(EEP)variable in the pressure delivery formula previously described) may beadjusted according to responses to a series of questions of which thegoal is to choose an EEP to minimize upper airway obstruction and unloadintrinsic PEEP. The methodology also detailed in the generalized flowchart of FIG. 5 includes a selecting step 50, a prompting step 52 and acalculating step 54. In the selecting step 50, an appropriate startingpressure value is defined. Based upon general ventilation-relatedcharacteristics that have assigned adjustment pressure values, forexample, 2 cmH₂O may be assigned to obesity, queries are formed in aprompting step 52. The final setting is calculated in a calculating step54, the setting is determined as a function of the initial pressurevalue and one or more of the assigned adjustment pressure values from aset of assigned adjustment pressure values representing the generalizedventilation related characteristics. Preferably, the assigned values areadded to the starting pressure value based upon the input responses inthe prompting step 52. In this final step, minimum setting limits may beenforced as a result of a particular classification of a patient'scondition. The preferred embodiment of the methodology is as follows::

[0129] Start at about 4 cmH₂O.

[0130] If the subject is sleepy (i.e., a state of a lack of wakefulnessof the patient), add about 1-2 cmH₂O.

[0131] If the subject is obese, add about 1-2 cmH₂O.

[0132] If the subject has a narrow upper airway, add about 1-2 cmH₂O.

[0133] If the subject has mild, moderate, or severe CAL, the finalpressure must be at least in a range of about 5-7 cmH₂O or about 5, 6,or 7 cmH₂O respectively.

[0134] The resultant EEP is in a preferred range of about 4-10 cmH₂O.

[0135] While this adjustment can be made manually, the device isoptionally automated to accomplish the above methodology. To this end,the device is programmed to accomplish the methodology by presenting aseries of questions on an output display to the subject/physician andprompt for input on an input device controlled by the microprocessor 16.Based upon the input responses the EEP may be adjusted automatically bycalculating and setting the appropriate EEP.

[0136] (4) Pressure Support (swing). Although the preferred device canbe set to have a minimum and maximum swing, it is preferred during thelearning period to set the maximum and minimum SWING sliders to the samevalue (i.e., no servo-adjust as yet), chosen to do about half of thepatient's awake elastic work. As with the setting determination for theEEP, the methodology for setting the SWING may be performed manually orautomated by the ventilator. To this end, the device may be programmedto accomplish the methodology by presenting questions on an outputdisplay to the subject/physician and prompt for input on an input devicecontrolled by the microprocessor 16. Based upon the input response theSWING may be adjusted automatically by calculating and setting theappropriate SWING.

[0137]FIG. 6 outlines the general steps in the methodology. In aselecting step 60, an initial or default swing pressure value is chosen.In a prompting step 62 responses to queries concerning degrees ofseverity of restrictive mechanical abnormality of lung or chest wall aregiven. In a calculating step 64, the swing pressure is determined as afunction of the initial pressure and a set of assigned adjustmentpressures that are assigned to degrees of severity of a restrictivemechanical abnormality in a preferred range of about 2-6 cmH₂O. Thedegrees may have multiple levels and the assigned values increase theinitial pressure by a fixed amount for each level of increase in thedegree of severity.

[0138] In the preferred embodiment, the methodology is as follows:

[0139] Start with about 3 cmH₂O.

[0140] For mild, moderate, or severe restrictive mechanical abnormalityof lung or chest wall (excluding neuromuscular or controlabnormalities), increase in a range of about 2-6 cmH₂O or by about 2, 4,or 6 cmH₂O respectively.

[0141] The resulting swing is in a preferred range of about 3 to 9cmH₂O.

[0142] (5) Backup Rate. The backup respiratory rate can be set to 5breath/min below the patient's expected respiratory rate. This does notneed to be at all accurate.

[0143] (6) Resistive unloading. Resistive unloading is preferably set todo about 50% to 80% of the patient's expected resistive work. As withprior settings, this preferred methodology for the resistive unloadingmay be performed manually or automated by the ventilator. To this end,the device may be programmed to accomplish the methodology by presentingquestions on an output display to the subject/physician and prompt forinput on an input device controlled by the microprocessor 16. Based uponthe input response the resistive unloading may be adjusted automaticallyby calculating and setting the appropriate value.

[0144]FIG. 7 outlines general steps in the methodology. In a queryingstep 70, the physician/user is prompted to determine whether the patienthas normal airway resistance or suffers from obstructive or restrictiveresistance. The prompting preferably assesses the degree of severity ofthe identified disease. In a setting the resistance step 72, based uponthe responses, assigned resistance values are used to set resistiveunloading. The assigned values represent different degrees of severityof the diseases, for example, normal airway resistance (i.e.,neuromuscular or control abnormalities), obstructive disease orrestrictive disease.

[0145] In the preferred embodiment of the invention, the methodology isas follows:

[0146] if the patient has normal airway resistance (e.g. neuromuscularor control abnormalities) start with a resistance of about 1cmH₂O/L/sec.

[0147] if the patient has mild moderate, or severe obstructive disease,set to a range of about 4-6 cmH₂O/L/sec. or about 4, 6, or 8cmH₂O/L/sec. respectively

[0148] if the patient has mild, moderate or severe restrictive disease,set to a range of about 3-8 cmH₂O/L/sec. or about 3, 4, or 8cmH₂O/L/sec. respectively. This compensates for the narrowed anddistorted airways at low volumes.

[0149] Thus, the preferred range of resistive unloading is in a range ofabout 1 to 8 cmH₂O/L/sec.

[0150] If the above setting of resistive unloading causes the patient tocomplain that the machine is “pushing them along”, or the pressure isoscillating during late expiration, reduce the resistive unloading.

[0151] (7) Duty Cycle (T_(I)/T_(TOT)). This setting is not very crucialand a value of 0.4 will suit most patients, because AutoVPAP quicklylearns the patient's duty cycle. However, for patients with moderate orsevere dynamic airway compression requiring very long expiratory times,a shorter duty cycle, say 0.3 or 0.2 could be used.

[0152] (8) Other settings. The other settings should be left at theirdefault values as follows: Wait Minimum Shape Maximum Servo Gain Maximum

[0153] 3. Summary of Initial Settings

[0154] Remember that the object of all the above settings is to unloadas much as possible of the patient's awake ventilatory work withoutmaking the patient uncomfortable due to excessive pressures.

[0155] The subject may now breathe on the device.

[0156] 4. Supplemental Oxygen

[0157] If necessary, supplemental oxygen is added to the mask, at up to4 L/min, in order to maintain awake arterial haemoglobin oxygensaturation at or above 90%.

[0158] 5. Learning Period

[0159] Once the subject is comfortable, and the mask has been checkedfor leaks, it is time to commence the “learning” period, which lasts 1hour. During this period, the subject is encouraged to watch televisionor read a book, is asked not to engage in conversation, to concentrateon the television rather than on breathing, and to avoid falling asleep.

[0160] To start the learning period, click on the FULL mode icon, whichis the right-most of the six mode icons at the top of the screen.

[0161] The device records the subject's spontaneous partially assistedventilation, for example, by determining minute volume, over a 1 hourperiod, and at the end of the hour automatically sets the targetventilation to equal 90% of the median ventilation during the final 40minutes. The first 20 minutes are discarded to permit the patient timeto settle and to become absorbed by the television program. The medianis chosen, rather than the mean, in order to be relatively immune totransients such as coughing or microsleeps. The one hour period servesthe dual function of learning the patient's spontaneous awakeventilation, and of acclimatizing the subject to therapy.

[0162] The time into the hour is displayed at the bottom right of thescreen. At the end of the hour, the device will automatically drop outof the learning mode, and back into the AutoVPAP treatment mode. Thepatient will not usually notice anything happen at this time. However,the screen will grey out for a period of ten seconds or so, and some ofthe sliders will move to new positions.

[0163] Once the learning period is over, most of the sliders will be inthe positions that you set prior to entering the learning period, withthe following exceptions:

[0164] Target ventilation will have been set to 90% of the medianventilation during the last 40 minutes of the learning period.

[0165] Backup rate will have been set to the median respiratory rateduring the learning period.

[0166] Maximum swing will have been set to 22 minus the EEP, which is ashigh as it will go.

[0167] Additional details concerning a learning period are the subjectof U.S. patent application Ser. No. 09/799,260 filed on Mar. 5, 2001,the disclosure of which is hereby incorporated by reference.

[0168] 6. Additional Adjustments

[0169] At the end of the learning period, the above settings should bereviewed, to make sure that they are sensible. Optionally, furtheradjustments to the settings may be made. At present, the only suggestedadjustment is to increase the target ventilation slightly in patientswho are struggling to maintain an adequate awake PCO₂. A flowchartsummarizing the steps in the methodology is depicted in FIG. 8. In adelivering step 80, ventilatory support is delivered to the patientduring a learning period. In a measuring ventilation step 82 andmeasuring PCO₂ step 84, patient ventilation related characteristics aremeasured. In a calculating step 86, a target is derived from themeasured ventilation. Finally, in an adjusting step 88, the calculatedtarget ventilation is adjusted by formulae which makes use of themeasured PCO₂. The preferred embodiment of the adjustment methodology isas follows:

[0170] For subjects with a daytime arterial PCO₂ above about 50 mmHg,increase target by about 1% per mmHg (e.g. 10% at 60 mmHg).

[0171] For subjects with a daytime arterial PCO₂ below about 50 mmHg,decrease target by about 0.5% per mmHg (e.g. 85% at 40 mmHg).

[0172] Of course, these adjustments may be made manually. Alternatively,the machine has programming instructions to automate the methodologyafter the learning period based upon measured or derived daytimearterial PCO₂ values. For example, the device makes the automatedmeasurements with the addition of apparatus to measure daytime arterialPCO₂ levels that provides data as input signals to the controller of thedevice. Alternatively, the device can prompt the user/physician to enterthe pertinent measurement data acquired by separate equipment. Anautomated apparatus for such measurements is disclosed in U.S. patentSer. No. 5,630,413. Upon entry or recording of the data, the devicecalculates the modified ventilation target as a function of the measureddaytime arterial PCO₂ and a threshold by either of the followingformulae depending on the value of the measured PCO₂:

V _(TGT-adjusted) =V _(TGT-learned)*[1+((|H−PCO₂|)*0.01)] (if PCO₂>H)

V _(TGT-adjusted) =V _(TGT-learned)*[1−((|H−PCO₂|)*0.005)] (if PCO₂<H)

[0173] Where:

[0174] PCO₂ is the measure of daytime arterial partial pressure of CO₂

[0175] H is a threshold value of preferably about 50 mmHg.

[0176] 7. Disconnecting and Switching Off

[0177] Once you have checked the final settings, the blower is now readyfor long-term home therapy.

[0178] The blower may be disconnected from the computer, the computerswitched off, and the blower switched off. This can be done in anyorder. The blower will remember the settings. It is not necessary todisconnect if you do not want to.

[0179] 8. Optional Awake Confirmation Period

[0180] If there is any clinical cause for doubt, the subject could bepermitted to continue for an additional hour at this new “treatmentmode” setting, and arterialized capillary blood PCO₂ or arterial PCO₂taken, to confirm that the subject is not being over-ventilated. Whilemanual measurements may be taken, the device may be optionally equippedto self test the patient's PCO₂ level. For example, an automated devicefor measuring PCO₂ as previously disclosed may be configured with thedevice to make a measurement during a testing mode following the first“treatment mode.” The measurement may be compared by the processor withacceptable levels of PCO₂ stored in the device. Those skilled in the artwill understand the PCO₂ levels that would indicate such overventilation. In response, the device may optionally issue an alarm orprevent further treatment if the comparison of the PCO₂ level indicatesover-ventilation.

[0181] Although the invention has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of an application of the principles of theinvention. Numerous modifications, in addition to the illustrativeembodiments of the invention discussed herein may be made and otherarrangements may be devised without departing from the spirit and scopeof the invention.

I claim:
 1. A method for determining a setting for a ventilator todeliver support to a patient to maintain a positive end expiratorypressure comprising the steps of: selecting an initial pressure value;prompting for responses to queries about a patient concerninggeneralized ventilation characteristics; and calculating a positive endexpiratory pressure from said initial pressure value and a set ofadjustment pressure values based upon said responses to said queries,said adjustment pressure values representing general ventilationcharacteristics.
 2. An apparatus for determining a setting for apositive end expiratory pressure and then delivering pressure support toa patient in accordance with the setting comprising: a means forproviding controlled pressurized air to a patient; an input means foraccepting data signals; an output means for displaying queries; acontroller operable to access data from said data signals and generateoutput through output signals to said output means, said controller withprogrammed instructions for executing a determination of a positive endexpiratory pressure, said instructions controlling the steps of:prompting for responses to queries about patient ventilationcharacteristics wherein said ventilation characteristics have assignedpressure values; and calculating a positive end expiratory pressure froman initial pressure value and said assigned pressure values based upondata from said data signal entered in response to said queries.
 3. Amethod for determining a setting for a ventilator to deliver support tomaintain a pressure swing in a specified range chosen to do about halfof a patients elastic work comprising the steps of: selecting an initialpressure value; prompting for a response to a query about a patientconcerning degrees of severity of a restrictive mechanical abnormalityof lung or chest wall; and calculating a pressure swing from saidinitial pressure value and a set of adjustment pressure values basedupon said response to said query, wherein said set of adjustmentpressure values represent degrees of severity of a restrictivemechanical abnormality of lung or chest wall.
 4. An apparatus fordetermining a pressure swing setting for a ventilator and to deliversupport at the setting to maintain a specified range chosen to do abouthalf of a patient's elastic work comprising: a means for providingcontrolled pressurized air to a patient; an input means for acceptingdata signals; an output means for displaying queries; a controlleroperable to access data from said data signals and generate outputthrough output signals to said output means, said controller withprogrammed instructions for executing a determination of a pressureswing, said instructions controlling the steps of: prompting forresponses to queries about degrees of severity of a restrictivemechanical abnormality of lung or chest wall wherein said degrees ofseverity have assigned adjustment pressure values; and calculating apressure swing from a base pressure value and said assigned adjustmentpressure values based upon data from said data signal entered inresponse to said queries.
 5. A method for determining a resistiveunloading setting for a ventilator to deliver support to unload about50% to 80% of a subject's resistive work comprising the steps of:prompting for a response to at least one query to determine thesubject's degree of severity of restrictive disease and obstructivedisease; and setting a resistive unloading value to one of a set ofassigned pressure values based upon said response to said at least onequery, wherein said set of assigned pressure values represents degreesof severity of restrictive disease and obstructive disease.
 6. Anapparatus for determining a resistive unloading setting for a ventilatorto deliver support to unload about 50% to 80% of a subject's resistivework and for delivering support at the setting comprising: a means forproviding controlled pressurized air to a patient; an input means foraccepting data signals; an output means for displaying queries; acontroller operable to access data from said data signals and generateoutput through output signals to said output means, said controller withprogrammed instructions for executing a determination of a resistiveunloading value, said instructions controlling the steps of: promptingfor a response to at least one query about degrees of severity ofrestrictive disease and obstructive disease wherein said degrees ofseverity have assigned pressure values; and setting a resistiveunloading value to one of said assigned pressure values based upon saidresponse to said at least one query.
 7. A method for determining atarget ventilation setting of a ventilator comprising the steps of:delivering ventilatory support during an awake learning period to apatient; measuring the patient's ventilation over time during a learningperiod; measuring the patient's partial pressure of CO₂; calculating atarget ventilation as a function of a measure of ventilation; andadjusting said target ventilation as a function of a measure of partialpressure.
 8. An apparatus for determining a target ventilation settingof a ventilator comprising: a supply means for providing controlledpressurized air to a patient; a flow means for generating a flow signalrepresentative of patient airflow; a PCO₂ means for generating a PCO₂signal representative of a measure of patient partial pressure of CO₂; acontroller operable to (a) access data from the flow signal and the PCO₂signal, and (b) to control the supply means, said controller withprogrammed instructions for executing a determination of a targetventilation setting, said instructions controlling the steps of:delivering ventilatory support during a learning period to a patient;determining a measure of patient's ventilation over time from saidlearning period; calculating a target ventilation as a function of themeasure of the patient's ventilation and as a function of the measure ofthe partial pressure of CO₂.